1. Field of the Invention
The present invention relates to an organic electroluminescent display, and more particularly, the present invention relates to an organic electroluminescent display that can prevent decreases in the average luminance thereof.
2. Description of the Related Art
An organic electroluminescent display is a display device for emitting light by electrically exciting fluorescent or phosphorescent materials. The organic electroluminescent display drives N×M number of organic electroluminescent display elements so as to display an image. As shown in FIG. 1a, the organic electroluminescent cell includes an anode (ITO), an organic thin film and a cathode (Metal). The organic thin film has a multi-layered structure including an emitting layer (EML) for emitting light with a combination of an electron and a hole, an Electron Transport Layer (ETL) for transporting electrons and a Hole Transport Layer (HTL) for transporting holes. The organic thin film may include an Electron Injecting Layer (EIL) for injecting electrons and a Hole Injecting Layer (HIL) for injecting holes.
An anode electrode is connected to a first voltage source to supply the holes to the light emitting layer (EML). A cathode electrode is connected to a second voltage source having an amplitude lower than that of the first voltage source to supply the electrons to the light emitting layer (EML). In other words, the anode electrode has a higher voltage potential of a positive(+) polarity and the cathode electrode has a lower voltage potential of a negative(−) polarity.
The hole transport layer (HTL) is supplied to the light emitting layer from the anode electrode by accelerating the supplied holes. The electron transport layer (ETL) accelerates the electrons supplied from a cathode electrode so that the electrons which are supplied from the electron transport layer (ETL) collide with the light emitting layer (EML). The electrons and holes are recombined in the light emitting layer (EML) so as to generate light. The light emitting layer (EML) is formed of organic materials and generates one of red (R) light, green (G) light or blue (B) light when the electrons and the holes are recombined.
In an organic electroluminescent element, negative carriers are located in the anode electrode of FIG. 1b, because a voltage supplied to the anode electrode is always set to be higher than that supplied to the cathode electrode. Positive carriers are located in the cathode electrode. If negative carriers in the anode electrode and positive carriers in the cathode electrode are kept for long time, the average luminance is decreased because the amount of movement of the electrons and holes is reduced.